Particulate emissions (black smoke) from diesel engines is significant. Diesel particulate material strongly absorbs light and leads to degraded visibility, particularly when there are several diesel-engine vehicles in an area. Diesel particulate material furthermore is easily respirated and is consequently of concern since it potentially includes mutagenic and carcinogenic chemicals. As a result of these and other reasons, various levels of governments regulate particulate emissions from diesel engines.
In response to the need to reduce engine particulate emissions, vehicle and engine manufacturers are attempting both to develop engines which produce cleaner exhaust and to develop particulate trap systems which clean the exhaust before emission to atmosphere. The latter approach is relevant to the present invention. The latter approach in general uses a device known as a trap-oxidizer. A trap-oxidizer system generally includes a temperature resistant filter (the trap) from which particulates are periodically burned off (oxidized), a process commonly known as regeneration. The traps must be regularly regenerated so as not to become excessively loaded and create an undesirable back pressure thereby decreasing engine efficiency.
Possible traps for capturing diesel particulate emissions primarily include cellular ceramic elements (see U.S. Pat. No. 4,276,071) and catalytic wire-mesh devices (see U.S. Pat. No. 3,499,269).
Trap-oxidizer regeneration systems can be divided into two major groups primarily on the basis of control philosophy. One group is positive regeneration systems; the other group is self-regeneration systems. Positive regeneration systems include the use of a fuel-fed burner (see U.S. Pat. No. 4,167,852), use of an electric heater (see U.S. Pat. No. 4,851,015) or use of techniques which aim to raise the temperature of exhaust gas temperature at selected times (see U.S. Pat. No. 4,211,075). Self-regeneration systems are directed, for example, to the use of catalytic treated traps to lower the ignition temperature of the captured particulates.
Currently, a popular trap is one which uses a cellular ceramic element and a popular regeneration method is one which uses a face electric heater to initiate regeneration of the trap. Although such trap and method can serve the particulate cleaning purpose well, cellular ceramic elements are subject to failure by cracking due usually to heat gradients caused by uneven burns, and experience with cellular ceramic/electric heater systems also makes it clear that it would be a distinct advantage to have a system which reduced the requirements of vehicular supplied power. The present invention, in its various embodiments, provides improved performance in these areas.